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A brighter future for LEDs

The light-emitting diode has long promised cleaner, cheaper light. New advances are making good on that promise.

February 26, 2007 |

By Jeff Yoders, Associate Editor

The light-emitting diode (LED) has come a long way and hasn't even reached its 45th birthday. Long the preferred lighting device for consumer electronics, solid-state LEDs (many tiny LED chips clustered together to form a light source) are starting to find acceptance in architectural and commercial lighting. The advantages of LEDs are that include operation on milliamps of electric current, they create almost no heat output, deliver an average of 32 lumens of light from a single watt of electricity, and they burn about 50 times as long as the average incandescent bulb. The Department of Energy estimates that LED lighting could reduce U.S. energy consumption by 29% by 2025, slicing $125 billion off our national energy bill in the process.

But the promise of the LED hasn't been fulfilled. LEDs are still too expensive for most applications due to the cost of the materials needed to build them. Currently, LEDs are still more expensive, in lumens per dollar, than more conventional lighting technologies. LEDs couldn't even produce white light until the early '90s, and even then the process didn't produce a bright, white light that doesn't have blue or yellow edges.

But advances in materials and energy efficiency over the last five years are finally unlocking the commercial and architectural lighting potential of the LED. A growing number of Building Teams are starting to adopt LEDs, especially for niche applications, where long life and the ability to operate with very little maintenance. Recent advances in university research have the potential to help LEDs to replace incandescent light bulbs in the next five to seven years.

“Most of the advances have come in light extraction from the chip,” said Dr. Nadarajah Narendran, director of the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, N.Y. “The problem has been that too much of the photon light created in a semiconductor chip has been stuck in the chip. Over the last three to five years we've seen increases in light output per chip. If we can get more light out of the chip, then the cost goes down in terms of how much light it produces. Then it can be used in more applications.”

Better light, same chip

Last June, Cree Lighting of Durham, N.C., reported results of 131 lumens per watt from a white LED source. The results were confirmed by the National Institute of Standards and Technology in Gaithersburg, Md. Tests were performed using prototype white LEDs with Cree EZBright LED chips.

“This is the highest level of efficacy that has been publicly reported for a white LED and raises the bar for the LED industry,” said Scott Schwab, Cree general manager of LED chips. Incandescent light bulbs typically use 10 to 20 lumens per watt and compact fluorescent bulbs usually range from 50 to 60 lumens per watt. If LEDs can consistently produce so much more light per watt, then they, indeed, have a bright future. Philips LumiLEDs of San Jose, Calif., and GELcore, based in Cleve-land, also have produced high lumen-per-watt white LEDs, but none with results as high as Cree's.

Why is it so important that the light be white? Because the color of an LED's light is determined by the materials creating the positive-negative electrical junction that produce it. LED manufacturers have struggled to find a proper material to create clean, white light comparable to what fluorescent and incandescent bulbs produce. Nichia Corp. of Japan first created white LEDs (using augmented light from blue chips) in 1993 using gallium nitride and indium gallium nitride, but almost no LEDs are produced using this method today. Most of today's white LEDs are produced by a blue light LED chip with phosphors that produce yellow light. When bonded together they produce a beam of white light. But even this solution doesn't produce combined, pure white light. You can still see the yellow and blue edges of the light sources. In June 2005, Philips LumiLEDs patented a proprietary phosphor conformal coating process that gives its LEDs a more consistent spectrum of white light for its Luxeon product line. The closely guarded bonding process eliminates the problem of varying phosphor thickness around the LED chips enabling Philips's LEDs to produce correlated color temperature measurements (a test of color variation of light sources) that show conventional white LEDs have roughly 15 times more color variation than white Luxeon emitters. The company has even started marketing its white LED products in a line of tiny chips, available at home-decorating stores, that customers can place in tinted candle holders to create the illusion of candlelight without the wax and fire hazard.

Solid uses for solid-state light

While cost is still a prohibitive factor for widespread LED adoption, the technology has been used for decades in consumer electronics and is now being embraced for high-visibility and high-energy consumption uses like grocery case lights.

Wal-Mart recently announced that it will outfit low- and medium-temperature refrigerated display cases in more than 500 of its U.S. stores with a GE ecomagination-certified LED system from GELcore, the LED business unit of GE. Wal-Mart is the first GELcore customer to roll LEDs out in a widespread application, but 20 other grocery customers, including eight of the top 10 chains in the nation, are already using ecomagination LED systems. Wal-Mart expects to net up to 66% energy savings from the retrofit. The savings are expected to come from occupancy sensors and dimming systems as well as electricity savings from the LED system itself.

Grocery chains such as Giant Eagle and Albertson's that are already using LEDs in grocery cases have said that LED light gives their products a better presentation than fluorescent light. Because LEDs produce less heat, grocers don't need to offset the heat they create with higher refrigeration energy costs.

Maintenance-intensive applications like traffic lights and signals have been turning to LEDs to save energy. Kentucky and Delaware currently use LEDs in all traffic signals statewide. Outdoor concert and art lighting designers often use LEDs because of their light direction capabilities.

“In addition to better phosphors, we're seeing more applications where the advantages of the LED, less maintenance, less energy, are being taken advantage of,” Narendran said. “With new technologies that are being developed right now, we're seeing advances that can make an impact in the market.”

Last year, Vanderbilt University graduate student Michael Bowers was asked to help a fellow researcher in the production of quantum dots, a special microscopic light-producing nanocrystal. Post-doctoral student and electron microscopist James McBride was interested in the way in which quantum dots grow. He thought that the structure of very small dots might provide him with new insights into the growth process, so he asked Bowers to make him a batch of small quantum dots that he could study. He sent Bowers back twice because the dots weren't small enough. Finally, Bowers made the smallest batch of quantum dots he knew how to create. The tiny quantum dots were relatively easy to make even though they are less than half the size of normal quantum dots. After Bowers cleaned up the batch, he pumped a solution containing the nanocrystals into a small glass cell and illuminated it with a laser. A white glow covered the table. The quantum dots were supposed to emit blue light, but instead they were giving off a beautiful white glow. The small size of the quantum dots that Bowers created turned out to be the perfect size to create white LED light.

The Vanderbilt researchers are the first to report making quantum dots that spontaneously emit white light, but they aren't the first to report using quantum dots to produce hybrid, white-light LEDs. Researchers at many universities are studying the commercial viability of quantum dot white LEDs.

University researchers are also studying the use of nanophosphors to achieve the same effect as quantum dots. Nanophosphors are just like the regular phosphors currently used to produce hybrid yellow-blue white LEDs, except that they're so small that they're only visible in the nanometer range. By interacting with LED substrates at that microscopic level, researchers believe nanophosphors can achieve the same effect as quantum dots because the properties of quantum dots and nanophosphors are so similar at that level. If nanophosphors can create less scattering and consistency of light in the LED package, they would significantly reduce the cost of spontaneous white LEDs because they would eliminate the expensive process of creating quantum dots.

Evident Technologies of Troy, N.Y., is working with the Lighting Research Center at Rensselaer Polytechnic Institute on a project funded by the New York State Energy Research and Development Authority to produce quantum dot-based LED lights.

“Part of what we're doing here [at the lighting research center] is setting realistic expectations,” Narendran, the RPI researcher, said. “The increases we've seen in light output per chip are certainly significant. When we can make an LED that has 100,000 or 200,000 hours of lifetime, it becomes equivalent to a building material.”

In 1962 Dr. Nick Holonyak, Jr., working for the General Electric Corp., invented a semiconductor diode device that emits light in the practical visible spectrum. LEDs consist of a chip, made of semiconductor material, impregnated with impurities to create a junction. Electrical current flows easily from the positive anode to the negative cathode of the junction but not in the reverse direction. Electrical charge-carriers, electrons, and electron holes flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases light energy in the form of a photon. In other words, when electrical current passes through them, LED chips illuminate, creating fast direct light that lasts much longer, about 50 times as long, and burns much brighter than conventional incandescent light bulbs.

Many researchers were working at the time on semiconductor advances using infrared light. In interviews Holonyak has said, “I wanted to work in the visible spectrum, I wanted to actually see the light.”

Holonyak is now the John Bardeen Endowed Chair Professor of Electrical and Computer Engineering and Physics at the University of Illinois at Urbana-Champaign, his alma mater. He worked with Bardeen, the only two-time winner of the Nobel Prize for physics, on semiconductor and superconductor technology throughout the late '80s. In 2002 Holonyak was awarded the National Medal of Technology.

Holonyak is still innovating. In 2005, a research center was set up by the Defense Advanced Research Projects Agency (DARPA) to develop Holonyak's and Illinois colleague Milton Feng's proposed LET, light-emitting transistor. LETs and transistor lasers enable optical and electronic functions to be integrated on a single chip and could lead to the development of optical computers that can run 1,000 times faster than today's electronic computers.